Egyptian supporters of deposed president Mohamed Morsi pray in front of barbed wire fencing that blocks the access to the headquarters of the Republican Guard in Cairo. (Mahmud Hams/AFP/Getty Images)

Our correspondent in Cairo, Abigail Hauslohner, sends this first-hand account of today's violence in the Egyptian capital, where military troops opened fire on supporters of ousted president Mohamed Morsi, killing at least 40 and injuring 300. Follow our live-blog for more on Egypt.

Pro-Morsi demonstrators who were eyewitnesses to the shooting said they were in the middle of the dawn prayer, just after 3 a.m., when gunshots rang out and tear gas canisters thudded into their midst.

Witnesses described a scene of confusion and chaos as live gunfire, birdshot and tear gas seemed to come from all sides. It was unclear where the first shot came from.

Abdel Naguib Mahmoud, a lawyer from the Nile Delta town of Zagaziz, said he and fellow protesters had knelt to the pavement for the second time during the early morning prayer, their backs to the Republican Guard palace, when he heard the shouts of other protesters from the sit-in’s perimeter, warning them that security forces were encroaching on their encampment.

“So we finished our prayer rapidly,” Mahmoud said, but before he could move, he heard the resounding boom of a tear gas shot and the crackle of gunfire. He said he ran with friends toward the entrance of the sit-in that had been manned by protesters, and found wounded there on the street.

Mahmoud said he and others began to pick up the wounded to carry them away when more shots rang out, so they lay down on the pavement. He claimed to see men in military fatigues and police in black. Moments later, an officer stood over him and kicked him, telling him to move, he said. When he stood up and ran, forces opened fire, shooting him more than a dozen times in the back with birdshot. He lifted his shirt to reveal a scattering of small bloodied wounds.

“This gave us even more resolve to defend our votes and the democratic process,” he said of the shooting.

Mahmoud and some other who were being treated at a makeshift field hospital Monday said they saw snipers on nearby rooftops as the attack took place. But accounts conflicted on which direction the first shots came from or who fired them. Witnesses only insisted that they came from the military.

“I don’t remember where we were facing, but the shooting came from everywhere,” said Abdel Rahman Mahmoud, a young metro cleaner, who sat on the concrete ground at the Brotherhood’s makeshift field hospital outside the Rabaa al-Adawiya mosque, both arms bandaged. “My friends were the ones who told me to run,” he said, sweat beading his forehead.

Muslim Brotherhood members and other supporters of the ousted president vowed Monday to continue their protests. Some called on the international community to intervene.

“This is a crime against humanity,” Mahmoud said. “We call on the international community to activate things at the International Criminal Court.”

Brotherhood members who spoke at an emotional news conference at the Rabaa al-Adawiya mosque, which lies at the heart of the group’s biggest sit-in in eastern Cairo, vowed to put General Abdel Fatah al-Sissi on trial.